This invention relates in general to semiconductor packaging and in particular to a leadframe with multi-tier leads.
With the development of very large scale integration technology, the size of the die used in semiconductor packages is continually being reduced. Even though the size of the die is reduced, it will be desirable to retain the capability of providing a large number of pins connected to each side of the die. In semiconductor packages employing leadframes, the die is supported by a die attach pad which forms a portion of the leadframe. The leadframe also has leads with lead tips close to the die for connecting selected points on the die to pins which extend outside the package.
The lead tips are typically close to the die so that bonding wires with lengths within an optimal range may be used for bonding the lead tips to the die. After wire bonding, the die, the die attach pad, the bonding wires and the lead tips are all enclosed by a molding material such as by plastic in an injection molding process. When the size of the die is reduced, and if the dimensions of the leadframe used for supporting the die remains the same, the edges of the die would be further apart from the lead tips. In such event, longer bonding wires would be required to bond the die to the lead tips.
The use of bonding wires whose lengths are longer than the optimal is undesirable since this would increase the stress at the portions of the wire where it is bonded to the die and to the lead tip, thereby increasing the probability of a faulty wire bonding connection. The alternative is to modify the leadframe by making the die attach pad smaller and by bringing the lead tips further inward towards the die so that bonding wires of optimal length can again be used to bond the die to the lead tips. In such case, the current state of the art leadframe design imposes a limit as to how close the lead tips can be brought towards the center of the die attach pad without having to reduce the number of leads provided for each side of the die as explained below.
The leadframe is made as follows. A flat sheet of metal is stamped to cut out portions of the sheet so that the portions remaining form the leadframe. In order to make individual leads, an essentially parallel array of thin slices of the sheet are cut out in the stamping process so that the long finger-shaped portions remaining form the leads. Alternatively, these slices can be removed in an etching process. In the current leadframe design, all the lead tips are in the same plane. A minimum spacing must be maintained between the lead tips to avoid electrical shorts.
In the current state of the art in stamping or etching technology, there are certain limits as to how small the width of the leads and as to how small the spaces between the leads can be. Therefore if the number of leads that are to be connected to each side of the die by wire bonds is to remain the same, the "design width" of spaces required between the lead tips in the current state of stamping or etching technology limits how close the leads can get to the die and the center of the die attach pad. This forces a designer to choose between reducing the number of leads that can be connected to each side of a die and using bonding wires that are longer than optimal. Either alternative is undesirable.
It is therefore desirable to provide a new leadframe system in which the above-described difficulties are alleviated.